BIOREACTOR
CONFIGURATIONS
• Usually, a cylindrical tank, either stirred or
unstirred.
• Reactor design
– Provision of adequate mixing and
– aeration for the large proportion of
fermentations requiring oxygen
Stirred Tank Reactors (STRs)
• Mixing and bubble dispersion, achieved by
mechanical agitation;
– High input of energy per unit volume.
• Baffles for reducing vortexing.
• Impellers for different flow patterns inside
fermentor
• Multiple impellers in tall fermentors, to
improve mixing
• 70-80% of the volume of stirred reactors is
filled with liquid
• Adequate headspace for
– disengagement of droplets from the exhaust
gas
– accommodating any foam which may develop
• A supplementary impeller – “foam breaker”
• Or, chemical antifoam agents added to
broth
– Reduces rate of O2 transfer
• Aspect ratio: Ratio of height to diameter
• Internal cooling coils: For temperature
control and heat transfer
• Used for free- and immobilised cells
Bubble Column Reactors
• No mechanical agitation
• Aeration and mixing, achieved by gas sparging.
– Requires less energy than mechanical stirring.
• Generally cylindrical vessels with height > twice
the diameter
• A sparger for entry of compressed air
• Typically, no internal structures.
• For industrial production of bakers’ yeast, beer
and vinegar (H:D ratio of ~ 3:1 to 6:1)
• For treatment of wastewater.
• Perforated horizontal plates to break up
and redistribute coalesced bubbles.
• Advantages
– Low capital cost,
– Lack of moving parts, and
– Satisfctory heat- and mass-transfer
performance
Airlift Reactors (ALRs)
• Mixing without mechanical agitation.
• For culture of plant and animal cells and
immobilised catalysts
– Because shear levels are much lower than in
STRs
• Patterns of liquid flow are more defined
• Physical separation of up-flowing and
down-flowing streams.
• Gas is sparged into part of the vessel
cross-section called Riser.
• Gas hold-up and decreased fluid density
cause liquid in the riser to move upwards.
• Gas disengages at the top of the vessel
leaving heavier bubble-free liquid to
recirculate through the downcomer.
• Liquid circulation is a result of density
difference between riser & downcomer.
• Internal-loop ALRs
– Riser and downcomer are separated by an
internal baffle or draft tube
– Air may be sparged into either draft tube or
annulus
• External-loop or outer-loop ALRs
– Separate vertical tubes, connected by short
horizontal sections at the top and bottom.
– Riser & downcomer are further apart in
external-loop vessels
• So, gas disengagement is more effective
• Density difference between fluids in the
riser and downcomer is greater
• So, circulation of liquid is faster.
• Thus, mixing is better in external-loop than
internal-loop ALRs
• In production of SCP
• For plant and animal cell culture
• In municipal and industrial waste
treatment.
• Height of ALRs is typically ~10 times the
diameter
• For deep-shaft systems, H / D ratio,
increased up to 100.
Packed Bed Reactors (PBRs)
• Used with immobilised or particulate
biocatalysts.
• Consists of a tube, usually vertical, packed with
catalyst particles.
• Medium can be fed either at the top or bottom of
the column
• Medium forms a continuous liquid phase
between the particles.
• Damage due to particle attrition is minimal
• Used for production of aspartate and fumarate,
conversion of penicillin to 6-aminopenicillanic
acid, and resolution of amino acid isomers.
• Operated with liquid recycle
• Catalyst is prevented from leaving the
column by screens at the liquid exit.
• Particles should be relatively
incompressible and able to withstand their
own weight in the column without
deforming and occluding liquid flow.
• Recirculating medium, to be clean & free
of debris to avoid clogging the bed.
PBR with medium recycle
Fluidised Bed Reactor (FBRs)
• Packed beds are operated in upflow mode with
catalyst beads of appropriate size and density
• Basis: Bed expands at high liquid flow rates due
to upward motion of the particles.
• Particles in fluidised beds are in constant motion
– Channelling and clogging of the bed are avoided
– Air can be introduced directly into the column.
• Used in waste treatment, brewing and for
production of vinegar.
Trickle Bed Reactor
• A variation of the packed bed
• Liquid is sprayed onto the top of the
packing
• Liquid trickles down through the bed in
small rivulets.
• Air may be introduced at the base
• Used widely for aerobic wastewater
treatment.